Porosity removal in certain ceramic materials such as silicon nitride based ceramics is difficult but yet such porosity removal is required for many uses of these ceramics such as cutting tools.
Hot isostatic pressing (HIPing) is an effective means of porosity removal. Because of the relatively high temperatures required for porosity removal and the complex geometrics required in the ceramic, various glasses are used as the encapsulating agent. The encapsulating agent converts the gas pressure of the HIP to physical pressure against the ceramic.
U.S. Pat. No. 4,446,100 and its divisional patent, U.S. Pat. No. 4,478,789, disclose that when a preformed ceramic powder body is subjected to isostatic pressing at the sintering temperature, it must, in order to give a desired dense product, be enclosed in a casing which, during the pressing, is able to prevent the pressure medium then used, normally a gas, from penetrating into the powder body. The casing, like its contents, is liberated from undesirable gases during a process stage prior to the sealing. Various ways of forming the casing are known.
According to U.S. Pat. No. 4,446,100, one method known prior to U.S. Pat. No. 4,446,100, a preformed capsule of glass is used as casing. In another known method, according to U.S. Pat. No. 4,446,100, the casing is manufactured on the spot by dipping the preformed powder body into a suspension of particles of glass, or surrounding it in some other manner with a layer of particles of glass and then heating it under vacuum at such a temperature that the particles form a tight casing around it. As far as silicon nitride is concerned, it is also known to use a porous layer of a glass of a low-melting type outside a porous layer of glass of a high-melting type. According to the above patent, the outer porous layer is transformed into a layer impermeable to the pressure medium while the powder body is degassed. When a tight layer has been formed, pressure is applied to the enclosed powder body by argon or helium to counteract dissociation of the silicon nitride when the temperature is continually raised. During the continued temperature increase, the glass in the outer layer reacts with the material in the inner porous layer while forming an increasingly high-melting glass and while maintaining a layer impenetrable to the pressure medium, and finally a glass layer which is impenetrable to the pressure medium is formed from the innermost part of the inner porous layer before the glass in the outer layer has time to run off. This last formed glass layer forms a casing around the powder body when the isostatic pressing thereof is carried out at the sintering temperature.
In the method which is the subject matter of U.S. Pat. No. 4,446,100, there is disclosed a method wherein a B.sub.2 O.sub.3 containing glass is used to obviate glass penetration into the silicon materials.
U.S. Pat. Nos. 4,446,100 and 4,478,789 and the disclosures thereof are incorporated herein by reference.
U.S. Pat. Nos. 4,462,816 and 4,462,817 and 4,462,818 all relate to various aspects of the manufacture of multiple silicon nitride articles using hot isostatic pressing. Each of these patents pertain to the hot isostatic pressing of more than one part at a time.
In these patents, two or more preforms are stacked together and are then encapsulated within a single capsule to be hot isostatically pressed. In order to prevent the preforms from diffusion-bonding to each other during hot isostatic pressing, suitable refractory spacers are placed therebetween. The spacers must be inert with respect to the silicon nitride preforms and must have melting point greater than the hot isostatic pressing temperature. If the spacer material, for example, alumina, is insufficiently inert with respect to the silicon nitride preforms, the alumina spacer may be coated with boron nitride to provide the desired inertness. In some cases it may be desirable to completely wrap the stack of preforms in a suitable metal foil prior to encapsulating the stack, in a capsule, the reason being to prevent any loose particles of powder that may be present on the stack from interfering with the formation of a sealed capsule.
Each of these patents and their respective disclosures incorporated by reference herein.
Use of glass in hot isostatic pressing, while effective for porosity removal from the ceramic, requires removal of adhered glass from the articles after the hot isostatic pressing step. Sometimes the removal is difficult, requiring machining or sand blasting of the pressed articles. Furthermore, in some instances some of the materials used may react with the material being pressed thereby creating undesirable second phases.
It is believed therefore a hot isostatic pressing process that will provide porosity removal and will enable the production of an article essentially free of adhered encapsulating material and of secondary phases would be an advancement in the art.